1. Field of the Invention
The present invention relates to an apparatus which may be employed in the extrusion of thermoplastic materials such as thermoplastic films for packaging, and the like. More specifically, an apparatus is provided which ensures uniform molten resin flow distribution through a tubular extrusion die, which minimizes resin stagnation in the die and which minimizes sensitivity of the die to different resins.
2. Description of the Prior Art
In the prior art techniques for the extrusion of thermoplastic tubular material, solid thermoplastic resin is fed into an extrusion apparatus wherein the resin is heated and becomes molten. The molten resin is continuously mixed and forwarded to the exit end of the extruder by a rotating screw device within the confines of the extruder barrel. The molten resin is then forced through an adaptor into a tubular extrusion die, and the molten resin is expressed from the die in the form of a tube. The extruded tube is drawn from the die, usually by a pair of positively driven nip rollers, which are located downstream from the die orifice. The tubular extrusion die may be fed with molten resin either from the center of its base, i.e. center-fed, or the die may be fed through an orifice located along the side of the die, i.e. side-fed. In the case of the center-fed design, the molten thermoplastic resin from the extruder is fed into the center of the bottom of the die, where it is divided and conducted radially through channels to positions around the die annulus. Such an arrangement provides for good uniformity of resin distribution around the interior cylindrical mandrel of the die. The resin distribution in the case of a center-fed die is superior to the distribution uniformity achieved in a conventional side-fed die apparatus.
A side-fed die design is frequently required however, for various mechanical configuration considerations including cases where a tubular die must have a continuous hole in its center, for example, where a rotating shaft must extend through the die to drive a machine within the film bubble, or where several tubes must extend through the die to conduct liquid in and out of the bubble; or in the case of coextrusion wherein a plurality of thermoplastic layers are fed through the die so that they all emerge in laminar relationship through a single annular outlet orifice on the die, the latter coextrusion methods and apparatus being well known to those skilled in the art.
A conventional side-fed die has unequal path lengths, thus unequal pressure drops, for the molten resin to travel to arrive at various positions about the periphery of the die mandrel, and for this reason, it is difficult to obtain good resin distribution. Without some compensating adjustment, more resin will flow to the side of the die nearest the feed port, since it is subjected to the lowest pressure drop.